LINQ’s Distinct() on a particular property

What if I want to obtain a distinct list based on one or more properties?

Simple! You want to group them and pick a winner out of the group.

List<Person> distinctPeople = allPeople
  .GroupBy(p => p.PersonId)
  .Select(g => g.First())
  .ToList();

If you want to define groups on multiple properties, here’s how:

List<Person> distinctPeople = allPeople
  .GroupBy(p => new {p.PersonId, p.FavoriteColor} )
  .Select(g => g.First())
  .ToList();

Note: Certain query providers are unable to resolve that each group must have at least one element, and that First is the appropriate method to call in that situation. If you find yourself working with such a query provider, FirstOrDefault may help get your query through the query provider.

Note2: Consider this answer for an EF Core (prior to EF Core 6) compatible approach. https://stackoverflow.com/a/66529949/8155